The present invention relates to lamp structure for controlling gas flow into the lamp outer envelope of an electric lamp during back-fill. The invention further relates to a lamp where such structure is an integral part of the lamp stem.
In electric lamps, the lamp stem extends into the interior space of the lamp outer envelope. An exhaust tube is connected to the lamp stem and communicates with the interior of the lamp envelope via a conduit through the stem wall. During the manufacture of electric lamps, the exhaust tube is used to evacuate the lamp envelope and to back-fill the lamp envelope with a predetermined gas. During the back-fill operation, the gas flows from a gas source through the exhaust tube and conduit into the interior of the lamp outer envelope. In the known lamp stems, the conduit has an opening adjacent the stem press and is generally straight. Gas exiting from the conduit during back-fill flows adjacent the stem press and impinges on the side interior wall of the outer envelope.
The interior surface of the outer envelope of electric lamps is often provided with a coating comprising powdered or particulate material. For example, incandescent lamps frequently have a coating or frosting on the inner surface of the lamp envelope to diffuse the light emanating from the filament. The familiar white frosted lamps have a coating of finely powdered white silica. Fluorescent lamps and some types of high pressure discharge lamps, such as metal halide, high pressure sodium and mercury lamps, have a coating of powdered phosphors on the inner surface of the lamp envelope. The phosphors convert ultraviolet radiation into visible light and modify the color rendition of the lamps.
In lamps having a coating on the interior of the lamp outer envelope, flow from the stem conduit as described above can have a velocity such that the gas impinges on the coating and loosens or blows the coating material off the wall. The thickness of the coating is then reduced or completely removed from a section of the envelope wall. The damaged coating will adversely affect appearance and possibly lamp performance. Such damaged lamps must then be rejected.
A solution to this problem has been to deflect the gas away from the envelope coating. In one arrangement, a metal shield is positioned over the opening of the conduit at an angle sufficient to deflect the gas flow away from the portion of the lamp envelope having the powdered or particulate coating. The metal shield is supported by a wire welded to the shield and to one of the lamp current supply conductors. Alternatively, U.S. Pat. No. 3,783,322 discloses a disc shaped metal heat shield which straddles the stem press and is positioned to deflect the gas away from the coating. However, these solutions require extra metal parts and additional manufacturing steps to attach the parts to the stem press or current-supply conductors, and add to the cost of the lamp. Moreover, the gas deflectors in the assembled lamps are frangible and are susceptible to vibration and shock damage.
Japanese Application No. 56-105002 discloses a lamp stem having a conventionally formed conduit for the exhaust tube which is further enlarged by drilling along the axis of the stem. The stem press has a concave portion circumscribing the drilled portion of the exhaust hole. The drilled portion of the exhaust hole and the curved stem press allows gas to flow into the envelope at an angle closer to the lamp axes than in conventional stems. However, gas flowing through the exhaust hole is directed toward the rounded end of the lamp envelope opposite the lamp stem which is an area normally provided with a coating. Additionally, the gas flow would impinge on the filament and possibly be deflected onto the coating on the side of the lamp envelope. Moreover, this stem requires the additional manufacturing step of drilling the glass stem.